The present invention relates to an optical system to be employed in an optical disc drive to read/write data from/to an optical disc.
Recently, technology in the field of magneto-optical disc drives has been greatly improved such that a data recording density on a magneto-optical disc has reached in excess of 10 Gbits/inch.sup.2.
In such an optical disc drive, an objective optical system is mounted on an arm which is movable in a transverse direction of tracks formed on an optical disc for rough tracking. Firstly, the rough tracking is performed to locate the optical head in the vicinity of the track. Then, the incident angle of a beam which is incident on the objective optical system is changed (i.e., a fine tracking is performed), with use of a deflector such as a galvano mirror or the like. During the fine tracking operation, the beam spot is accurately located on one of the tracks, whose pitch is, for example, 0.34.mu.m.
In order to control the tracking operation, it is necessary to detect the amount of rotation of the galvano mirror. Specifically, in the optical disc drive described above, if the galvano mirror is rotated to a position out of a certain rotational range to adjust the position of the beam spot, optical performance of the disc drive may be significantly lowered. Thus, the rotation angle of the deflection mirror should be monitored and controlled so as not to exceed a predetermined rotational range.